#include <iostream>
#include <vector>
using namespace std;
struct TreeNode
{
    int val;
    TreeNode *left;
    TreeNode *right;
    TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};

class Solution
{
public:
    vector<int> preorderTraversal(TreeNode *root)
    {
        
    }
};